I sent to a calibration baseline yesterday, and used two Leica GPH1P prisms. I don't have a manual or anything, and I was using them with a Trimble SX10. When I solve for the prism constant, I get -33.5 mm±0.5 mm. I had thought it was -34, so that looks correct. But, I found a prism offset table online, and it says these are 0 offset.
I think I heard that Leica EDM (never used one) are setup for -34, which would make sense in the table above that an actual -34 offset would be 0.000, and a reflective tape would be +34.4 mm
Is my assumption correct?
Also, I was very surprised that the SX10 I was using has a -5.1 ppm scale error (±0.71 ppm). I was very careful about T and P, and this was on pillars, so no centering error. The temperature was measured with Omega RH83, and checked with the nearest weather station. In order to account for a -5.1 ppm error due to temperature, it would have to have been 6.5°C (11.7°F) lower, and I am sure I was within 1°. The SX10 is spec'd at 1 mm±1.5 ppm.
The way the Trimble S, VX, and SX series total stations work is they have a value stored in the instrument for the refractive index and also the carrier wavelength, and those values are stored in the job file for processing. The actual distance stored in a job file is with no offset applied, and 0 ppm. So I believe the J value (refractive index) stored in the instrument is incorrect. I believe this value can be changed by the dealer in the shop, but I don't know how good (i.e. accurate) their reference baseline is. I have a high accuracy S6 (which is in the shop waiting for a part from China) that has always tested on the CBL at better than 1 ppm.
I process all total station data outside of TBC, in a database that reads, stores, and applies the J and N values along with measured T and P to get the corrected distance. I do it this way so that I can use temperature and pressure collected with an external sensor after the fact and not worry about changing it in the field as it warms up or cools down. I believe I can just change the J value to reflect the calibration determined scale factor by adding 5.1 to 269.899 (the stored J value).
I am going to a different CBL to confirm. Hopefully I will get the S6 back soon and can take it as well, although I am not hopeful because of everything going on.
Yes, that is correct - for Leica prisms you must subtract 34.4mm from the factory-stated offset. 0.0mm "Leica offset" equals -34.4mm absolute offset.
Concerning the ppm error, is there any chance that the copper prism coating is affecting the EDM measurements? IIRC the Leica coating was specifically developed for their TS carrier frequencies. There is a white paper on it that I used to have, can't find it on my machine. Trimble prisms do not have such a coating.
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When I go the the alternate baseline I will take some of our standard prisms we use daily, which are seco mini prisms (-29) and try them alongside the Leica. I have a two pedestal baseline on my property, but it is only 130 m long so not really good for detecting scale bias but I will compare them there as well. We go to the CBL every spring, and then measure the local two pedestal line right after that. I have a personal weather station right next to my baseline, so that makes it easier to get accurate atmospheric corrections.
This is from a Leica newsletter regarding the majority of Leica prisms that have a copper coating.
The front surface of Leica prisms has an anti-reflection coating. This coating is remarkably hard and has the added feature of protecting the surface from scratches. Without this, the front face of a prism will reflect part of the EDM signal. At close ranges, this causes incorrect distance measurements.
The coating is optimised for the Leica EDM signal. Therefore other brands of coated prisms will still partially reflect and can cause incorrect measurements.
The Leica GPH1P prisms do not have any coating on the glass. This is from the same newsletter.
An exception is the GPH1P precision reflector. Although this prism has no antireflex coating, the prism is mounted at a slight tilt to prevent any direct reflection back to the EDM.
Here is a data sheet that has the dimensions of Leica prisms.
I had sent these to you earlier, but for completeness I've included them here too. Look forward to hearing how your test at the second baseline comes out.
Thanks Gene. This is the first time I have used those on a baseline. I really like the fact that the carriers are 0.196, same as the instrument
Yeah, I've always appreciated the small design things that Wild/Leica does to keep me from screwing up anything.
The one odd thing to get used to is that a Leica 0.0 mm prism offset is really 34.4 mm.
